The present invention relates generally to a wide angle zoom lens system, and more particularly to an ultra-wide angle zoom lens system that is designed for use on cameras, etc.
In order that compact cameras which serve multiple functions but in which weight is given on portability are used in enlarged photographic ranges, zoom lenses are generally built in them. A zoom lens best known for this purpose has a standard focal length, as expressed in terms of a field angle of about 47.degree., and achieves a field angle change of about 65.degree. to about 35.4.degree. in association with zooming. In order that such cameras are used in more enlarged photographic ranges, not a few zoom lenses having increased wide angle or telephoto zooming zones are now being designed. However, the wide angle zooming zone is smaller in extent than the telephoto zooming zone. Moreover, even with cameras with the so-called wide angle zoom lenses built in them--in which the focal length change is limited to the wide angle zone, the field ranges at the wide angle end lie chiefly at about 65.degree. to about 72.degree.. These zoom lenses are not said to well cover photographic ranges for panoramic photography, and so much is desired as to wide angle functions. Zoom lenses having a field angle exceeding about 72.degree. are larger in field angle changes, when compared with focal length variations, and so are greatly promising as zoom lenses that have some remarkable effect on image formation even at a small zoom ratio. For instance, a zoom lens that changes in focal length from 21 to 35 mm has a zoom ratio as low as 1.67 but shows a large-enough field angle change of about 92.degree. to about 65.degree., and so seems to be promising in a sense of enlarging a photographic range.
Generally, a zoom lens to be incorporated in a compact camera is designed as a two-unit type making use of a so-called telephotographic type of positive and negative power arrangement, as viewed from the object side, in order to reduce the overall lens length. In order to enable this type of two-unit zoom lens to achieve a well-enough back focus, however, it is required that the positive power of the first lens unit be increased and the space between the principal points of the second lens unit be reduced as well, causing the first lens unit that is large in the heights of on-axial rays to produce excessive aberrations. This renders it very difficult to make a bright lens system, and particularly limits the F-number at the telephoto end to 5.6 or more. When it is intended to reduce the focal length at the wide angle end and thereby enlarge the field angle, it is necessary to increase the positive power of the first lens unit further and decrease the negative power of the second lens unit. This renders it more and more difficult to achieve a well-enough back focus, and makes the angle of inclination of off-axial rays incident on the first lens unit so large that the first lens unit generates excessive off-axial aberrations. Moreover, the angle of inclination of off-axial rays leaving the first lens unit is so large that the outer diameter of the second lens unit is imperatively increased.
For the reason mentioned above, it is proposed to dispose an additional lens unit of negative power on the object side of the two-unit type of positive and negative zoom lens to make an arrangement of three, say, negative, positive and negative powers, thereby increasing the aperture of the lens system and achieving a wide angle zooming zone, as typically set forth in JP-A-63-25613 and 64-72114. In one example of the former, there is disclosed a lens system that is bright, as indicated by an F-number of about 4.5 at the telephoto end. However, the focal length at the wide angle end is limited to about 35 mm. In one example of the latter, a lens system having a zoom ratio increased to about 3 is disclosed. However, this system is dark at the telephoto end, as indicated by an F-number of 5.5 or more, and the focal length at the wide angle end is limited to about 28 mm.
A typical zoom lens having a field angle at the wide angle end that is enlarged to about 92.degree. is set forth in Example 4 of JP-A-3-208004. The system comprises, in order from the object side, a first lens unit of positive refracting power, a second lens unit of negative refracting power, a third lens unit of positive refracting power and a fourth lens unit of negative refracting power, and uses a very large number of, say, 14 lenses with a complicated zooming mechanism. Hence, this has an overall lens length so long and varies so considerably in the location of the entrance pupil during zooming, that the first lens unit is increased in outer diameter. Thus, this lens system does not only lend itself fit for use in a compact camera, but is also dark at the telephoto end in particular, as indicated by an F-number of 4.5 to 5.6.